1. Field of the Invention
The present invention relates to a radial magnetic bearing apparatus for suspending a magnetic object by controlling a magnetic force with electromagnets, and, particularly, to the structure of control magnetic poles of the electromagnets.
2. Description of the Prior Art
FIG. 1 is a sectional view of a radial magnetic bearing apparatus of the prior art. A control magnetic pole core 1 is provided with control magnetic poles 31, 32, 35 and 36 around which control coils 51, 52, 55 and 56 are wound. An object made of a magnetic material, for example, a rotating shaft 8, is suspended in a direction X under a control force of electromagnets by control magnetic poles and control coils. The generated control magnetic poles 31 and 32 form a pair. The respective control coils 51 and 52 are wound to form different magnetic poles so that the magnetic fluxes formed by these coils are closed. Therefore, a pair of control magnetic poles 31 and 32 attract the rotating shaft 8 in the +X direction. On the other hand, a pair of control magnetic poles 35 and 36 provided opposite to the control poles 31 and 32 also attract the rotating shaft 8 in the -X direction. As a result, the rotating shaft 8 is positioned and supported in the direction X without any contact with the control magnetic poles 31, 32, 35 and 36 by balancing the magnetic force generated by the electromagnets. In the same manner, the rotating shaft 8 is also positioned and supported in the direction Y without any contact with the control magnetic poles 33, 34, 37 and 38 by balancing the magnetic force generated by the electromagnets comprising a pair of control magnetic poles 33 and 34 and a pair of control poles of 37 and 38.
A pair of control poles located at a position opposite to a pair of control poles provided with control coils supports the rotating shaft 8 in a balanced position under a magnetic force generated by these electromagnets. As such, it is necessary to control excitation of the control coils by detecting the position of the rotating shaft, in order to balance the magnetic forces generated by the electromagnets. To this end, an inductance-type displacement sensor is usually provided adjacent to each control magnetic pole to detect the radial position of the rotating shaft 8. This sensor has a magnetic pole similar to the control magnetic pole and around which a sensor coil is wound for detecting a change in inductance.
FIG. 2 is a sectional view of inductance-type displacement sensors provided in the radial magnetic bearing apparatus shown in FIG. 1. The inductance-type displacement sensors comprise sensor magnetic poles 41 to 48 and sensor coils 61 to 68 wound around the sensor magnetic poles. The position of the rotating shaft 8 in the direction X can be detected by measuring a ratio of inductance of a pair of sensor coils 61 and 62 wound around the magnetic poles 41 and 42, respectively, and the inductance of a pair of sensor coils 65 and 66 wound around the magnetic poles 45 and 46 located opposite to the magnetic poles 41 and 42 with respect to the rotating shaft. The position of the rotating shaft 8 in the direction Y is detected by measuring a ratio of the inductances of the sensor coils 63, 64, 67, 68 in a similar manner.
FIG. 3 is a sectional view of the radial magnetic bearing apparatus shown in FIG. 1. As shown in FIG. 3, displacement detecting points of the inductance-type displacement sensor comprising the sensor magnetic pole 41 around which the sensor coil 61 is wound and the inductance-type displacement sensor comprising the sensor magnetic pole 45 around which the sensor coil 65 is wound are located by .DELTA.L below in the axial direction of the rotating shaft 8 from working points of the control magnetic pole 31 and the control magnetic pole 35, respectively.
However, such a conventional magnetic bearing apparatus having control magnetic poles and magnetic poles of inductance-type displacement sensors, when operated at a level above the intrinsic value of bending vibration of the rotating shaft 8 results in oscillation which is impossible to control. FIG. 4 is a diagram explaining a bending mode vibration at the intrinsic value of bending vibration of the rotating shaft 8. In FIG. 4, positions AA' and CC' indicate the position of the working point of the control magnetic poles, while positions BB' and DD' indicate the position of poles of the displacement sensors. The control magnetic pole positions AA' and CC' are displaced by .DELTA.L from the magnetic pole positions BB' and DD' of the displacement sensors as shown in FIG. 3.
Under a normal control condition, the rotating shaft 8 is straight as indicated by symbol 8a, but under conditions of vibration occurring at the bending intrinsic value, the shaft 8 is bent as indicated by symbol 8b. Therefore, since each displacement sensor detects a displacement at the position separated by .DELTA.L from the position of the control magnetic pole under vibration occurring at the bending intrinsic value, the displacement sensor may erroneously indicate that a control force is required to be exerted upwardly, although a control force exerted downwardly is essentially required at the operating point of the control magnetic pole. In such a case, since an exciting current flows into the control magnetic pole to generate a control force exerted upwardly, the rotating shaft 8 is driven to an oscillating condition, resulting in a loss of control on the magnetic bearing apparatus.
Such oscillating and no control conditions may be avoided by conforming the position of the operating point of each control magnetic pole with the position of a corresponding magnetic pole of the displacement sensor. However, conventional displacement sensors consist of magnetic sensors, except for optical sensors or electrostatic capacitance-type sensors. Accordingly, if an inductance-type displacement sensor is mounted near a control magnetic pole, the displacement sensor generates an error signal by magnetic fields generated by the control pole.
On the other hand, an optical sensor is very expensive and is not desirable from the point of view of cost. Moreover, since control magnetic poles generate heat due to exciting currents, electrostatic capacitance-type sensors which are easily influenced by a change in temperature can not be employed.